1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer, or a facsimile machine, and more specifically to an image forming apparatus which forms an image with a two-component developer including toner and carrier, and to a toner supply control method for the image forming apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus, upon receipt of an input of an image forming job, such as a print job, a latent image carrier is driven to rotate and an outer circumferential surface of the rotating latent image carrier is uniformly charged to a target charge electric potential. Thereafter, the outer circumferential surface of the latent image carrier is exposed to light according to image information, thereby forming an electrostatic latent image on the outer circumferential surface of the latent image carrier. Then, due to a development potential, i.e., a potential difference between the electrostatic latent image on the latent image carrier and an outer circumferential surface of a developer carrier, the toner in the two-component developer (hereinafter simply referred to as the developer) on the developer carrier electrostatically adheres to the electrostatic latent image, thereby developing the electrostatic latent image into a toner image. The toner image on the latent image carrier formed by the development process is ultimately transferred onto a recording medium, thereby forming an image thereon. In such an image forming apparatus, a change in temperature or humidity, or deterioration over time of the developer, causes a change in toner charge in the developer, resulting in a change in development γ (gamma). As illustrated in FIG. 1, the development γ corresponds to the slope of a straight line representing the relationship between the development potential and the toner adhesion amount per unit area of the electrostatic latent image, i.e., per unit latent image area.
The development γ may be detected by the following method: That is, predetermined electrostatic latent images (i.e., latent patterns for image density adjustment) are first developed with different development potentials to form toner images (i.e., toner patterns for image density adjustment). Then, respective toner adhesion amounts of the toner images are detected by a toner adhesion amount sensor, and the slope of a straight line representing the relationship between the development potential and the toner adhesion amount based on the detection results is the development γ.
If the development γ changes, toner images resulting from electrostatic latent images developed with the same development potential have different toner adhesion amounts, and thus have different image densities. Further, in a color image formed by toner images of a plurality of colors superimposed upon one another, respective image densities of the colors individually change, and thus color reproducibility deteriorates.
Accordingly, the image forming apparatus may be configured to execute a process control, i.e., an image density adjustment control involving detecting the development γ at predetermined intervals, such as at every formation of a predetermined number of images, for example, and adjusting one or more development conditions, such as a target charge potential of the latent image carrier, a development voltage applied to the developer carrier, or exposure power used in the latent image forming process, to obtain a target image density.
Further, in the image forming apparatus using a two-component developer including toner and carrier, if the toner concentration in the developer is not appropriately controlled, the target image density is not obtained. Normally, therefore, the toner concentration in the developer of a development device is detected as appropriate, and supply of toner to the development device is controlled such that the detected toner concentration approaches the target toner concentration.
If the development γ is substantially changed in the image forming apparatus which executes the image density adjustment control, the above-described adjustment of the development condition by itself may still result in degraded image quality. For example, if the development condition is adjusted by the image density adjustment control in accordance with the substantially changed development γ, the development potential may be excessively reduced. In this case, if the image forming apparatus executes an image gradation control by changing the exposure power, defective gradation is caused. Conversely, if the development potential is excessively increased by the image density adjustment control, the development voltage needs to be increased beyond what a development power source may be able to supply unless a high-capacity power source is provided, which increases the cost. Moreover, if the development potential is excessively increased by the image density adjustment control, adhesive force of the toner adhering to the outer circumferential surface of the latent image carrier is increased by a relatively strong electric field in the development process, resulting in a transfer failure.
In the above-described process control performed by the image forming apparatus, as well as the adjustment of the development condition, a process of adjusting the target toner concentration is also executed if the development γ substantially deviates from a target value even when the toner concentration is adjusted to the target toner concentration. For example, if the development γ is smaller than the target value by 0.3 mg/cm2·V even when the toner concentration is adjusted to a target toner concentration of 5 wt %, a process of adjusting the target toner concentration to 7 wt % is executed. With this control, it is possible to obtain the target image density by changing the toner concentration while adjusting the development potential within a range not causing the above-described undesirable phenomena.
As well as the above-described image density adjustment control, a forced toner consumption control involving forcibly consuming depleted toner in the development device and replacing the depleted toner with new toner may be executed as one way to adjust the image density. In this case, toner consumption control is executed to replace the depleted toner in the development device with new toner, thereby preventing the shortage of the toner charge and providing the target image density. Normally, toner remaining for a relatively long time in the development device without being used in the development process is continuously affected by a developer mixing operation performed in the development device, and thereby gradually deteriorates and suffers degraded charging performance. If the proportion of such depleted toner in the development device is increased, insufficiently charged toner is also increased. As a result, the toner adhesion amount per unit latent image area is excessively increased, and an image density higher than the target image density is obtained.
In the foregoing image forming apparatus, which changes the target toner concentration to adjust the image density, if the print job has a substantial change in, for example, a toner consumption index value, such as the image area ratio of images to be formed, the image density or color reproducibility of the post-change image may be degraded.
As a specific example, in a print job of continuously forming images having a relatively low image area ratio (e.g., approximately 1%), the toner consumption by image formation is relatively small. Therefore, a relatively large amount of toner unused in the development process remains in the development device, as a result of which overcharged toner continuously affected by the mixing operation and having a toner charge larger than usual is increased in the development device. In this case, the image density is reduced. Therefore, a control of increasing the target toner concentration to increase the toner concentration in the developer is executed, to thereby suppress a reduction in image density and maintain the image density during the print job.
If such a print job of continuously forming images having a relatively low image area ratio is followed by a print job of continuously forming images having a relatively high image area ratio (e.g., approximately 100%), the target toner concentration is still set to a relatively high value at the beginning of the latter print job, and an actual toner concentration is higher than usual. If the print job of images having a relatively high image area ratio starts in this state, much of the overcharged toner in the development device is soon consumed by the development process of the image forming operation of the images having a relatively high image area ratio. Then, in a relatively short period of time taken to form a few to dozens of images, most of the toner to be used in the development process is replaced by newly supplied toner having an appropriate toner charge.
FIG. 2 is a graph illustrating transition of the toner concentration in a print job of continuously forming images having a relatively high image area ratio that follows a print job of continuously forming images having a relatively low image area ratio. FIG. 3 is a graph illustrating transition of the image density in this case. In FIGS. 2 and 3, broken lines indicate the results of a related-art image forming apparatus having the foregoing configuration which changes the target toner concentration to adjust the image density, and solid lines indicate the results of a later-described image forming apparatus according to an earlier application of the present inventor.
At a point ta in FIG. 2, at which most of the toner to be used in the development process has been replaced by newly supplied toner having an appropriate toner charge, as described above, the toner concentration has barely changed from the toner concentration at the end of the previous print job, i.e., from the toner concentration adjusted for the overcharged toner, and is substantially higher than a toner concentration appropriate for the toner having the appropriate toner charge, as indicated by the broken line in FIG. 2. After the point ta in a period A, therefore, a shortage of the toner charge is caused by an excessive amount of toner in the developer, and the image density is higher than a target image density, as indicated by the broken line in FIG. 3.
Meanwhile, after the start of the print job, a control of forming predetermined toner patches, detecting respective toner adhesion amounts thereof, and adjusting the target toner concentration based on the detection results is executed at every formation of, for example, ten images. In the print job of images having a relatively high image area ratio following the print job of images having a relatively low image area ratio, a control of reducing the target toner concentration is executed in the target toner concentration adjustment control after the start of the print job to correct the state in which the image density is relatively high, as described above. Therefore, the toner concentration in the developer of the development device is gradually reduced, as indicated by the broken line in FIG. 2. Then, the time elapses to a point near the end of the period A, at which the toner concentration has been reduced to a certain level, and a relatively large amount of overcharged toner unused in the development process and continuously affected by the mixing operation remains in the development device owing to the continuing state of excess of the toner amount. As a result, the toner used in the development process contains a relatively large amount of overcharged toner, causing an insufficient image density in a period B, as indicated by the broken line in FIG. 3.
As described above, in the image forming apparatus which changes the target toner concentration to adjust the image density, if two consecutive print jobs are substantially different from each other in, for example, the image area ratio, a substantial change in image density occurs in the latter print job, making it difficult to maintain the target image density.
However, the present inventor has found that, if a print job of images having a relatively high image area ratio follows a print job of images having a relatively low image area ratio, as in the above-described specific example, phenomena such as unstable image density, stain in a non-image area, and toner scattering occur during the print job, particularly in an early stage of the print job, even if the control of adjusting the toner concentration to the value suitable for the relatively high image area ratio of the print job has been executed before the print job. Further, the present inventor has found from investigations that these phenomena occur due to the following reason.
FIG. 4 is a graph plotting toner charge distribution by size of charge in the development device, obtained when the image forming apparatus according to the earlier application adjusts, after a print job of images having a relatively low image area ratio, the toner concentration to a value suitable for a relatively high image area ratio of the next print job, and then executes the print job of images having the relatively high image area ratio.
During the print job of images having a relatively low image area ratio, the toner consumption by the image formation is relatively small, as described above. Therefore, a relatively large amount of toner remains in the development device, without being used in the development process. As a result, overcharged toner continuously affected by the mixing operation and having a toner charge larger than usual is increased in the development device. Further, the toner affected by the mixing operation is degraded in charging performance, and acts as weakly charged toner having a toner charge smaller than usual, or acts as inversely charged toner charged to a polarity opposite to a target polarity. In a state in which the toner amount consumed by the development process is relatively small, as in the print job of images having a relatively low image area ratio, the toner concentration is increased by the target toner concentration adjustment control executed during the print job, and thereby the toner amount in the developer is increased. Accordingly, the amount of depleted toner, such as weakly charged toner and inversely charged toner, is particularly increased.
The image forming apparatus can be configured to adjust the toner concentration after the print job of images having a relatively low image area ratio. This adjustment of the toner concentration, however, does not result in a reduction of the depleted toner in the development device. As illustrated in FIG. 4, a relatively large amount of depleted toner, such as weakly charged toner and inversely charged toner, is present in the developer of the development device for a while after the start of the subsequent print job of images having a relatively high image area ratio. If a relatively large amount of weakly charged toner is present in the developer, the toner adhesion amount per unit latent image area changes, making it difficult to stabilize the image density at the target image density. Further, if a relatively large amount of inversely charged toner is present in the developer, phenomena such as stain in a non-image area and toner scattering occur.
The above-described phenomena are not limited to the case in which a print job of images having a relatively low image area ratio shifts to a print job of images having a relatively high image area ratio, and may similarly occur, in different degrees, in a case in which the print job has a substantial change in, for example, the toner consumption index value, such as the image area ratio, including a case in which a print job of images having a relatively high image area ratio shifts to a print job of images having a relatively low image area ratio.